Adrenergic regulation of sodium and chloride transport in the isolated cornea of rabbit and man

Serotonin in Human Tears

It has been postulated that serotonergic receptors are present in the corneal epithelium and that their activation by serotonin released from subepithelial corneal nerves raises the level of cyclic AMP, which in turn stimulates active CI-secretion by the corneal epithelium. We looked for serotonin in tears because these bathe the corneal epithelium. Twenty-two normal subjects, 14 women and 8 men between 25 and 60 years of age (average 36.5 +/- 9 years), participated in this study. Twenty microliters of tears were collected in capillary tubes after trigeminal stimulation (pepper in the nose) and were immediately analysed using high-performance liquid chromatography with electrochemical detection. Serotonin was identified and measured in every subject. Its mean concentration was 2.74 +/- 1.99 ng/ml. No difference was found between women and men. Emotionally evoked tears were obtained from two subjects. The amount of serotonin in these samples was much higher than in the samples obtained by trigeminal stimulation. These results demonstrate for the first time that serotonin is present in human tears. The possible role of this serotonin in chloride transport in the corneal epithelium and the probable difference between "emotional" and stimulated tears warrants further investigation.

Localization of beta1-adrenergic receptors in the cochlea and the vestibular labyrinth

Sympathetic activation in a "fight or flight reaction" may put the sensory systems for hearing and balance into a state of heightened alert via beta1-adrenergic receptors (beta1-AR). The aim of the present study was to localize beta1-AR in the gerbil inner ear by confocal immunocytochemistry, to characterize beta1-AR by Western immunoblots, and to identify beta1-AR pharmacologically by measurements of cAMP production. Staining for beta1-AR was found in strial marginal cells, inner and outer hair cells, outer sulcus, and spiral ganglia cells of the cochlea, as well as in dark, transitional and supporting cells of the vestibular labyrinth. Receptors were characterized in microdissected inner ear tissue fractions as 55 kDa non-glycosylated species and as 160 kDa high-mannose-glycosylated complexes. Pharmacological studies using isoproterenol, ICI-118551 and CGP-20712A demonstrated beta1-AR as the predominant adrenergic receptor in stria vascularis and organ of Corti. In conclusion, beta1-AR are present and functional in inner ear epithelial cells that are involved in K+ cycling and auditory transduction, as well as in neuronal cells that are involved in auditory transmission.

Corneal nerves: structure, contents and function

This review provides a comprehensive analysis of the structure, neurochemical content, and functions of corneal nerves, with special emphasis on human corneal nerves. A revised interpretation of human corneal nerve architecture is presented based on recent observations obtained by in vivo confocal microscopy (IVCM), immunohistochemistry, and ultrastructural analyses of serial-sectioned human corneas. Current data on the neurotransmitter and neuropeptide contents of corneal nerves are discussed, as are the mechanisms by which corneal neurochemicals and associated neurotrophins modulate corneal physiology, homeostasis and wound healing. The results of recent clinical studies of topically applied neuropeptides and neurotrophins to treat neurotrophic keratitis are reviewed. Recommendations for using IVCM to evaluate corneal nerves in health and disease are presented.

Chloride concentrations in the corneal epithelium and stroma from different species

Using ion chromatography, the concentration of chloride in the corneal stroma and epithelium of ox, pig and rabbit was measured to detect possible species differences. In all three species, the chloride concentration in the stroma was found to be very similar, i.e. 363 +/- 14 mmol kg-1 dry wt in ox, 364 +/- 18 mmol kg-1 dry wt in pig and 358 +/- 24 mmol kg-1 dry wt in rabbit. In contrast, a substantially higher concentration of 232 +/- 33 mmol kg-1 dry wt was found in the corneal epithelium of rabbit in comparison to the values determined in the same tissue of ox, i.e. 137 +/- 4 mmol kg-1 dry wt and of pig, i.e. 128 +/- 24 mmol kg-1 dry wt. Using the results obtained previously for the extracellular volume (Midelfart 1988), an intracellular chloride concentration of 25 +/- 2 mmol kg-1 H2O was calculated in the bovine corneal epithelium. The results obtained in this study indicate that species differences are considered as important in studies of ion transport mechanisms in the cornea.

Sympathetic innervation of the rat cornea as demonstrated by the retrograde and anterograde transport of horseradish peroxidase-wheat germ agglutinin

The sympathetic innervation of the rat cornea was studied by using the method of intraaxonal transport of horseradish peroxidase-wheat germ agglutinin conjugate (HRP-WGA). In the first set of experiments, the relative number of superior cervical ganglion neurons that innervate the rat central cornea was estimated by the method of retrograde HRP-WGA transport. Following tracer application to the scarificed central corneal surface, 49-198 labeled neurons were observed in the ophthalmic region of the ipsilateral trigeminal ganglion and zero to four cells in the rostral pole of the ipsilateral superior cervical ganglion. In the second set of experiments, the three-dimensional distribution and termination sites of the corneal sympathetic nerve fibers were investigated by the technique of HRP-WGA anterograde transport from the superior cervical ganglion. HRP-WGA-labeled axons in corneal whole mounts were identified by the presence within their axoplasm of linear arrays of HRP-TMB reaction product, and their distribution was plotted faithfully onto line drawings made with a drawing tube attachment. Large numbers of HRP-labeled fibers were found in all animals within the corneoscleral limbus where the majority were associated with blood vessels. Fewer fibers (zero to 14 per animal) entered the cornea proper. The latter fibers entered the peripheral cornea in the deep to middle layers of the stroma and ascended into progressively more superficial layers as they coursed centrally. The majority of fibers branched infrequently in the peripheral cornea and increased in branching complexity near the central cornea. HRP-labeled axonal varicosities suggestive of terminal and preterminal expansions were located preferentially in the subepithelial layer of the corneal stroma and in the basal epithelium. Approximately 75% of the axonal varicosities were located in the central half of the cornea. The results of the current investigation reveal that the rat cornea is innervated sparsely by sympathetic nerve fibers derived from the superior cervical ganglion. These data provide additional support to current theories that corneal sympathetic nerve fibers may influence select aspects of corneal physiology, including ion transport and hydration, mitogenesis and wound healing, and sensitivity.

The effects of amiloride, ouabain and osmolality on sodium transport across bovine cornea

Electrical parameters across different parts of bovine cornea were measured after varying osmolality, temperature, pH and sodium concentration of bathing solutions, with and without amiloride or ouabain. It is concluded, that the transcorneal potential difference represents the sum of the transendothelial and transepithelial potential differences. Amiloride completely and reversibly inhibited the potential difference and the short circuit current originating in the epithelium (Ki = 2 X 10(-6) mol/l). Furthermore, the transcorneal electrical resistance increased by 28 +/- 8%. The results were used to calculate the cellular and paracellular pathway resistances in the corneal epithelium. An equivalent electrical circuit for sodium transport across the bovine cornea is proposed, which simulates the results obtained.